That is not hard to answer: The poet Cairpre Mac Edaine of the Tuatha Dé Danann.

What was the cause of this satire?

That is not hard to answer. The poet came seeking hospitality to Bres. At that time, they say, Bres was the sovereign of Ireland. Now this Bres Mac Eladain of the Tuatha Dé Danann had the best figure in Ireland. To him was compared every great and beautiful thing that each man sees on earth: a warrior is Breslike and great, and a fire is great and Breslike.

But the men of Ireland were not pleased with the rule of Bres, for he forced the champions into servitude. The two men who in Ireland were the strongest of the Tuatha Dé Danann, he put into slavery, namely Ogma Mac Eladain and the Dagda: Ogma (he placed) under a load of firewood, and the Dagda (he set) at ditch-digging, so that he entrenched the fortress of Bres. From day to day, then, Ogma used to supply the household with a load. The men of Ireland bore malice and ill-will towards Bres. Never did either man or woman go from him drunk or happy.

Now the poet Cairpre Mac Edaine came seeking hospitality to his dwelling. He was conveyed into a small outlying house which was narrow, dark, and dim, and there was neither fire, nor bath, nor bed. Three small cakes, and they [are] dry, were brought to him on a little dish. On the next day, lie arose, and he was not pleased. As lie went out across the court, Cairpre said:

“Without food speedily on a platter,
Without a cow’s milk whereon a calf thrives,
Without a man’s habitation after the staying of darkness,
Be that the luck of Bres Mac Eladain”.

“Not exists now Bres’s wealth”, he said.

This came to be true; for from that time forth his riches did not continue, so that the men of Ireland expelled him from his kingship. On account of his maternal relationship he had obtained the rule over the men of Ireland, and his father was a Fomorian. Now after being ejected, he went to his paternal relatives, the Fomorians, and he assembled them against the men of Ireland, and the Battle of Moytura was fought against the Tuatha Dé Danann. In this combat a slaughter overtook the Fomorians, so that few of them escaped, and so that henceforth no Fomorian came into the land of Ireland. In this conflict Bres Mac Bladain himself fell.
Now this, they say, is the first satire which was ever pronounced in Ireland, even the satire which Cairpre Mac Bdaine made upon Bres Mac Eladain.